1. Field of the Invention
The present invention relates to a slip control system for a vehicle and, more particularly, to a slip control system for the vehicle, adapted to prevent a slip of the driven wheels on a road surface from becoming excessive.
2. Description of Related Art
Prevention of a slip of the driven wheels from becoming excessive at the time of accelerating or the like is effective in order to efficiently provide a propulsive force of the vehicle as well as to provide safety in terms of prevention of a spin of the vehicle body and so on. In order to prevent an excessive amount of a slip of the driven wheels, torque to be applied to the driven wheels as a cause of slipping may be reduced.
Slip control of this type is disclosed, for example, in Japanese Patent Unexamined Publication (kokai) Nos. 16,948/1983 and 56,662/1985. Further, Japanese Patent Unexamined Publication (kokai) No. 231,836/1987 discloses a vehicle equipped with a torque converter having a lockup clutch interposed between an engine and a driven wheel, in which the lockup clutch is arranged so as to be forcibly uncoupled during slip control. In this technology known to the art, in instances where the lockup clutch is uncoupled during the slip control, the torque converter can reduce its efficiency of transmission of torque to the driven wheel by a portion at which the torque converter slips, and it can reduce a shock resulting from a variation in torque to be applied to the driven wheel due to its fluid buffer action, such a variation being caused by the slip control.
However, the slip control system of this type wherein the lockup clutch is uncoupled during slip control may be likely to cause the problem that a large extent of a slip may occur again immediately after the end of the slip control. In other words, if the lockup clutch is coupled in synchronization with the end of the slip control, torque to be applied to the driven wheel is temporarily increased due to no slip of the torque converter, thereby causing a large extent of a slip at the driven wheel again. This phenomenon may cause a hatching in which the startup and end of the slip control is frequently repeated as well as result in a frequent repetition of coupling and uncoupling the lockup clutch. These matters are not preferred from a viewpoint of durability.